There have been proposed a number of image-reading photosensors for use as an element member in various information processing devices or copying machines.
For instance, in the transmission of facsimile or in a copying machine, there is employed a photosensor having the function of reading images such as manuscript. A representative example in this respect wherein such image-reading photosensor is used is schematically illustrated in FIG. 3, in which are shown image-reading photosensor 301, self-focusing light transmitting body 302 such as SELFOC LENS (registered trademark name of NIPPON GLASS SHEET CO., LTD.) being disposed downward to the photosensor, two light emitting diode (LED) arrays 303, 303 respectively being arranged beside the light transmitting body and a manuscript 304 to be read.
There are known a number of kinds of such photosensors. Among these known photosensors, those wherein a non-monocrystalline semiconducting thin film such as an amorphous semiconducting thin film or polycrystalline semiconducting thin film is used as the photoelectric conversion layer are commonly considered preferable from the view points that it has a wealth of practically applicable photoelectric conversive characteristics and it can be easily sized to a large square measure.
In the known image-reading photosensor using such non-monocrystalline semiconducting material as the photoelectric conversion layer the photoelectric conversion layer composed of a non-monocrystalline semiconducting material is disposed on an electroinsulative substrate. And along with those image-reading photosensors, there have been proposed various methods for preparing the photoelectric conversion layer using vacuum evaporation, ion plating, reactive sputtering, thermo chemical vapor deposition, plasma chemical vapor deposition and photo chemical vapor deposition techniques. Among these methods, the method using plasma vapor deposition (hereinafter referred to as "plasma CVD method") has been generally recognized as being the most preferred and is currently used to manufacture the photoelectric conversion layer.
However, for any of the known photoelectric conversion layers, even if it is an acceptable one that is obtained by the plasma CVD method and exhibits almost satisfactory characteristics, there still remain problems unsolved in totally satisfying the requirements for its characteristics, particularly electric and optical characteristics, photoconductive characteristics, deterioration resistance upon repeated use and use-environmental characteristics, its homogeneity, reproducibility, mass-productivity, and its long term stability and durability, which are required for the photoelectric conversion layer to be a stable one.
The reasons are largely due to the fact that the photoelectric conversion layer cannot be easily prepared by a simple layer deposition procedure but skilled manipulations are required in the process operations in order to obtain a desirable photoelectric conversion layer while having due regard for the starting materials.
For example, in the case of forming a film composed of an amorphous silicon material (hereinafter referred to as "a-Si") according to the thermo chemical vapor deposition technique (hereinafter referred to as "CVD method"), after the gaseous material containing silicon atoms is diluted, appropriate impurities are introduced thereinto and the thermal decomposition of related materials is carried out at an elevated temperature between 500 .degree. and 650.degree. C. Therefore, in order to obtain a desirable a-Si film by the CVD method, precise process operation and control are required, and because of this the apparatus in which the process according to the CVD method is practiced becomes complicated and costly. However, even in that case, it is extremely difficult to reproduceably obtain on an industrial scale a desirable homogeneous photoelectric conversion layer composed of an a-Si material having practically applicable characteristics.
Now, although the plasma CVD method is widely used nowadays as above mentioned, it is still accompanied with problems relating to process operations and to capital investment.
Regarding the former problems, the operating conditions to be employed under the plasma CVD method are much more complicated than the known CVD method, and it is extremely difficult to generalize them.
That is, there already exist a number of variations even in correlated parameters concerning the temperature of the substrate, the amount and flow rate of gases to be introduced, the degree of pressure and the high frequency power for forming the layer, the structure of the electrodes, the structure of the reaction chamber, the flow rate of gases to be exhausted, and the plasma generation system. Besides said parameters, there also exist other kinds of parameters.
Under these circumstances, in order to obtain a desirable deposited film product it is required to choose precise parameters from a great number of variables. And sometimes serious problems occur. For instance, because of the precisely chosen parameters, a plasma is apt to be in an unstable state which causes problems in the deposited film.
Also, the structure of the apparatus in which the process using the plasma CVD method is practiced will eventually become complicated since the parameters to be employed are precisely chosen as above stated. Whenever the scale or the kind of the apparatus to be used is modified or changed, the apparatus must be so structured as to cope with the precisely chosen parameters.
In this regard, even if a desirable deposited film should be fortuitously mass-produced, the film product becomes unavoidably costly because (1) a heavy investment is firstly necessitated to set up a particularly appropriate apparatus therefor ; (2) a number of process operation parameters still exist even for such apparatus and the relevant parameters must be precisely chosen from the existing various parameters for the mass-production of such films. In accordance with such precisely chosen parameters, the process must then be carefully practiced.
Against this background, image-reading photosensors have become commonplace nowadays. There thus is an increased demand to stably provide a relatively inexpensive image-reading photosensor having a photoelectric conversion layer with a normal square measure or a large square measure being composed of an a-Si material which has a good uniformity and many practically applicable characteristics and which is suited for its use purpose and application.
Consequently there is an earnest desire to develop an appropriate method and apparatus to satisfactorily meet the above demand.
Likewise, there is a similar situation which exists with respect to other kinds of non-monocrystalline semiconducting layers which may constitute the photoelectric conversion layer of an image-reading photosensor, for example, those composed of an a-Si material containing at least one selected from oxygen atoms, carbon atoms and nitrogen atoms.